1. Filed of the Invention
The present invention relates to a cast wheel for a vehicle, and more particularly relates to a wheel wherein a rim portion and a disk portion are integrally cast.
2. Description of the Prior Art
In the field of a wheel for a vehicle, a large number of so-called full face type wheels, which can obtain a good design of a disk by increasing the surface area of the disk, have been produced. Usually the full face type wheel is manufactured by casting a rim portion holding a tire and a disk portion integrally connected to the rim portion.
The conventional full face type cast wheel is constructed, for example, as shown in FIGS. 9 and 10. A wheel 1 is formed by integrally casting a rim portion 2 and a disk portion 7. Surface 7a of the disk portion 7 is continuously connected to one of the flange portions 6 of the rim portion 2.
In such a cast wheel, since the surface 7a of the disk portion 7 extends to the flange portion 6 of the rim portion 2 in the radial direction of the wheel with a substantially flat configuration, a portion 10 becomes thicker in the axial direction of the wheel in comparison with other types of wheels. This portion 10 is surrounded by the surface 7a, a wall surface 3a of a side wall portion 3 constituting a rising wall from a dropped portion 2a to a hump portion 4 of the rim portion 2, the hump portion 4, a bead seat portion 5 to which a bead portion of a tire is attached, and the inside surface of the rim portion. Thus, unnecessary cast metal tends to increase in the portion 10. This existence of the unnecessary metal is contrary to a requirement of lightening a wheel.
Disk portion 7 usually has a plurality of spoke portions 8 extending in the radial direction of the wheel. In the example shown in FIG. 9, the portions between adjacent decorative openings 9 are constituted as spoke portions. The existence of the unnecessary metal in the portion 10 as dissussed above causes blowholes (holes due to contraction) to be liable to occur in the spoke portions 8 during manufacture of the wheel. When the wheel 1 is cooled after the wheel is cast using molds, the portion 10 having unnecessary cast metal tends to draw cast metal which is not yet hardened from the portions surrounding the portion 10, especially from the spoke portions 8. This portion 10 draws unhardened metal because the contraction of the portion 10 is large as a result of the large volume of the portion 10 as compared with that of the surrounding portions. As a result, blowholes are liable to occur in the spoke portions 8. The blowholes in the spoke portions 8 decrease the strength of the spoke portions themselves and the strength of the connection between the rim portion 2 and the disk portion 7. This deteriorates product yield.